Ozone and organic nitrates over the eastern United States: Sensitivity to isoprene chemistry


Mao, Jingqiu, et al. “Ozone and organic nitrates over the eastern United States: Sensitivity to isoprene chemistry”. J. Geophys. Res. Atmos. 118 (2013): , 118, 11,256–11,268. Web.






We implement a new isoprene oxidation mechanism in a global 3-D chemical transport model ({GEOS-Chem).} Model results are evaluated with observations for ozone, isoprene oxidation products, and related species from the International Consortium for Atmospheric Research on Transport and Transformation aircraft campaign over the eastern United States in summer 2004. The model achieves an unbiased simulation of ozone in the boundary layer and the free troposphere, reflecting canceling effects from recent model updates for isoprene chemistry, bromine chemistry, and {HO2} loss to aerosols. Simulation of the ozone-{CO} correlation is improved relative to previous versions of the model, and this is attributed to a lower and reversible yield of isoprene nitrates, increasing the ozone production efficiency per unit of nitrogen oxides ({NOx} ≡ {NO} + {NO2).} The model successfully reproduces the observed concentrations of organic nitrates ({∑ANs)} and their correlations with {HCHO} and ozone. {∑ANs} in the model is principally composed of secondary isoprene nitrates, including a major contribution from nighttime isoprene oxidation. The correlations of {∑ANs} with {HCHO} and ozone then provide sensitive tests of isoprene chemistry and argue in particular against a fast isomerization channel for isoprene peroxy radicals. {∑ANs} can provide an important reservoir for exporting {NOx} from the {U.S.} boundary layer. We find that the dependence of surface ozone on isoprene emission is positive throughout the {U.S.}, even if {NOx} emissions are reduced by a factor of 4. Previous models showed negative dependences that we attribute to erroneous titration of {OH} by isoprene.